1. Field of the Invention
The present invention relates generally to spin echo apparatus and more particularly to a spin echo system which is capable of detecting and distinguishing between an ECM deception signal and a true skin radar return from a target being struck by RF pulses transmitted from a radar system.
A spin echo phenomenon normally exists when a paramagnetic sample is located in a resonant cavity situated in a homogeneous DC magnetic field such that when a first or "input" RF pulse having a frequency equal to the characteristic or "Larmor" frequency of the sample is applied with its RF magnetic field substantially at right angles to the DC magnetic field whereupon a torque is applied to the magnetic moment of the sample which causes it to be tipped away from the direction of the magnetic field. The angle of tipping, that is the angle between the moment and the direction of the field is proportioned to the magnitude of the field and the time during which the RF pulse exists. Upon release of the displacing force, the spinning electrons urged again towards realignment by the force of the magnetic field rotate or precess about the field in much the same manner as a tipped gyroscope. When a sample is subsequently subjected to another or "recall" RF pulse whose RF magnetic field is also directed normal or transverse to the magnetic field, the sample spontaneously develops a magnetic field of its own which is also normal to the magnetic field and which rotates about the latter's direction. The strength of the rotating field builds up to a maximum and then decays which is then detected as an electrical pulse called a "spin echo signal".
2. Description of the Prior Art
Pulsed nuclear induction spin echo systems are well known to those skilled in the art, being taught for example by U.S. Pat. No. 2,887,673 issued to E. L. Hahn. Electron spin echo systems, moreover, are taught in U.S. Pat. No. 3,129,410 issued to P. P. Sorokin. Additionally, a system for generation of an echo similar to a spin echo signal without the need for an external magnetic field has been developed by the assignee of the present invention, utilizing a very heavily doped sample of paramagnetic material such as titanium dioxide (rutile) doped with iron group transition metal ions wherein the doping concentration is in the order of 1 .times. 10.sup.19 to 1 .times. 10.sup.21 defects centers per centimeter .sup.3. This concept is taught in U.S. Pat. No. 3,671,855 entitled "Broadband Zero Field Exchange Echo System". Nickel doped rutile as a spin echo sample obviating the need for an external magnetic field is disclosed in U.S. Ser. No. 33,201 entitled "High Bandwidth-Time Product Spin Echo System". The latter invention is also assigned to the assignee of the present invention.
The present invention has for its object the application of spin echo techniques to radar technology and is directed to means for determining the use of an ECM (electronic countermeasure) jammer used against a bi-frequency pulse radar system and comprises coupling a portion of two radar RF pulses, one at an instantaneous carrier frequency f.sub.1 and the other at a frequency f.sub.2 with the frequencies being separated by a difference frequency .DELTA.f where .DELTA.f &lt;&lt; f.sub.1 and f.sub.2 to a nonlinear element which provides a composite RF return having intermodulation product frequencies substantially the same as an ECM jamming equipment would produce. For example a resistive nonlinearity will give rise to third-order intermodulation product frequencies equal to 2f.sub.2 - f.sub.1 and 2f.sub.1 - f.sub.2. The RF pulse containing the intermodulating products is coupled to a spin echo system tuned to the center frequency of the composite RF pulse. A wideband post-set pulse is locally generated a predetermined time delay thereafter and is also fed to the spin echo system whose post-set pulse sets the spin system and produces a first spin echo signal which is not seen at the output of the spin echo system due to the fact that a detector is gated off during this interval. During the radar receive time, the target return signals are fed to the spin echo system. Since a target skin radar return signal does not contain intermodulation product frequencies, a correlation spin echo signal will not be produced, however, an ECM return signal from the target will contain the aforementioned intermodulation product frequencies which when coupled into the spin echo cavity will produce a correlation spin echo signal after a time substantially equal to the time difference between the intermodulation frequency RF pulse fed to the spin echo system and the post-set pulse. During the latter time interval, however, the detector is gated "on" and is responsive to the correlation spin echo signal produced by the ECM return signal thereby indicating that a countermeasure system is operating against the radar.